In recent years, there has been an increasing interest in organic semiconducting materials as an alternative to conventional silicon-based semiconductors. In particular, formulated organic semiconducting materials which comprise in part a polymer component have several advantages over those based on silicon, such as lower cost, easier manufacturing, solution processability at low temperatures as well as increased device flexibility, mechanical robustness, good compatibility with a wide variety of flexible substrates and light weight. They thus offer the possibility of producing more convenient high performance electronic devices.
Polyacene compounds and their analogues in particular have shown promise in this field of technology. WO 2005/055248 for example, discloses an organic semiconducting layer formulation comprising an organic binder which has a permittivity (ε) at 1000 Hz of 3.3 or less, and a polyacene compound. However the method for preparing the OFETs described in WO 2005/055248 in practice is limited and is only useful for producing top gate OFETs having relatively long channel lengths (typically >50 microns). A further disadvantage of WO 2005/055248 that is overcome by the present invention, is that it frequently uses undesirable chlorinated solvents. The highest performance semiconductor compositions disclosed in WO 2005/055248 having mobilities ˜1.0 cm2V−1s−1, incorporated 1,2-dichlorobenzene as the solvent (page 54, Table 5 and examples 14, 21 and 25). Moreover these solvents are not ones that would be industrially acceptable in a printing process and these are also damaging to the environment. Therefore it would be desirable to use more benign solvents for the manufacture of these semiconductor compositions.
Furthermore, it is generally thought that only polymer binders with a permittivity of less than 3.3 could be used since any polymers with a higher permittivity resulted in a very significant reduction in mobility values of the OFET device.
This reduction in mobility value can further be seen in WO 2007/078993 which discloses the use of 2,3,9,10-substituted pentacene compounds in combination with insulating polymers having a dielectric constant at 1000 Hz of greater than 3.3. These formulated organic semiconductor compositions are reported to exhibit mobility values of between 10−2 and 10−7 cm2V−1s−1 which are too low to be industrially useful.
Therefore, the present invention seeks to provide organic semiconductor compositions, which overcome the above-mentioned problems, by providing copolymers that are soluble in non-chlorinated organic solvents, have tunable permittivity values determined by the ratios of the defined copolymers and which when used in combination with polycrystalline small molecule organic semiconductors afford OSC formulations having sufficiently high solids loadings (>1% total solids), high flexibility and high mobility values in both top gate and bottom gate TFT devices.